The present invention generally relates to communication signal processing, and particularly relates to processing amplitude-modulated communication signals.
Communication signals as used in typical wireless communication systems commonly convey information via time-varying modulations in their frequency, or phase, or amplitude, or in some combination of those parameters. For example, the Wideband Code Division Multiple Access (W-CDMA) standards define a high-speed downlink channel referred to in the past as the High Speed Downlink Packet Access (HSDPA) channel, but more recently referred to as the High Speed Downlink Shared Channel (HS-DSCH). Transmissions on the HS-DSCH use either Quadrature Phase Shift Keying (QPSK) or 16-ary Quadrature Amplitude Modulation (16QAM), depending on the data rate needed.
At the receiver, turbo decoding generally is used, at least for decoding traffic symbols modulated according to the 16QAM format. Since turbo decoding depends on having accurate “soft” symbol information, the receipt of amplitude-modulated traffic symbols imposes significant signal processing challenges. More particularly, the receiver generally must have an amplitude reference for the received traffic signal, so that the as-received amplitude of the traffic signal can be properly referenced for symbol detection and soft information determination.
Obtaining an amplitude reference for the received traffic signal may be achieved by transmitting additional information to the receiver, but the additional signaling overhead required for that approach represents just one of the disadvantages of such schemes. Other approaches to determining an amplitude reference for the traffic signal involve relatively complex calculations, and/or involve assumed or modeled values relating the traffic and pilot signals. In the first case, maintaining a reasonably accurate amplitude reference imposes significant computational overhead on the receiver's signal processing elements. In the second case, the amplitude reference calculated by the receiver is no better than the accuracy or appropriateness of the assumed or modeled values.
Of course, the above challenges are not limited to WCDMA-based communication networks, or to 16QAM signal formats. For example, similar challenges may arise in the context of cdma2000-based systems, at least for certain signaling formats defined by the cdma2000 air interface standards. More generally, the ability to determine accurate amplitude references is needed by all receivers adapted for the demodulation of signals having higher-order amplitude modulation formats, or otherwise adapted for high data rate applications where soft value information is needed for accurate symbol decoding.